An electric power steering apparatus for applying a steering assist force to a steering mechanism of an automobile by a rotating force of a motor transmits a driving force of the motor by a transmitting mechanism such as a gear or a belt through a reduction gear to apply the steering assist force to a steering shaft or a rack shaft. An example of a brief structure of such an electric power steering apparatus is shown in FIG. 1 and will be described.
A shaft 102 of a steering wheel 101 is coupled to tie rods 106 of steered wheels through a reduction gear 103, universal joints 104a and 104b, and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 for detecting steering torque of the steering wheel 101, and a motor 108 for assisting a steering force of the steering wheel 101 is coupled to the shaft 102 through the reduction gear 103.
It is very important to detect a rotation angle θ of the motor for controlling the motor 108 of the electric power steering apparatus mentioned above, and in the case that any abnormality is generated in the angle detection, it is necessary to immediately detect the abnormality and take a proper safety measure. Accordingly, various detecting methods and detecting apparatuses have been conventionally developed in connection with the abnormality detection.
For example, Japanese Patent No. 3136937 discloses a detection abnormality of an angular resolver corresponding to one of the angle detecting device, and contents thereof will be explained with reference to FIG. 2.
An apparatus for detecting an angle is constituted by an angular resolver 10, and an angle detecting process portion for calculating a rotation angle θ by processing a signal output from the angular resolver 10. As one example of the angle detecting process portion, a resolver digital conversion (RDC) circuit 14 is provided. Further, a subject to be detected the abnormality is constituted by the angular resolver 10 except the resolver digital conversion circuit 14, a wiring to the angular resolver 10 or the like, and a detecting method is generally executed by detecting an abnormality of the angular resolver 10 by detecting an abnormality of a sin signal (=sin ωt·sin θ) and a cos signal (=sin ωt·cos θ) corresponding to the output signal of the angular resolver 10.
In this case, the resolver digital conversion circuit 14 has a structure provided with a function of detecting the abnormality of the angular resolver 10, and an abnormality detecting signal is output from an E terminal of the resolver digital conversion circuit 14 in FIG. 2. However, since the performance of detecting the abnormality is definite and is not sufficient, there is a case that a unique abnormality detecting circuit explained below is provided.
In FIG. 2, a carrier wave oscillation circuit 12 sends a carrier wave signal sin ωt to the angular resolver 10 for detecting a rotation angle θ of a motor 108, whereby a sin signal (sin ωt·sin θ) and a cos signal (sin ωt·cos θ) corresponding to the rotation angle θ are output from the angular resolver 10. The rotation angle θ is calculated in the resolver digital conversion circuit 14 to which the sin signal and the cos signal are input, the calculated rotation angle θ is incorporated into a CPU circuit 16, and the motor 108 is controlled on the basis of the rotation angle θ.
A description will be given of a state in which the abnormality of the resolver 10 or the like is detected by using the sin signal and the cos signal with reference to a flow chart in FIG. 3.
As a method of calculating the sin θ from the sin signal, a peak value of the sin signal is held by sampling the sin signal in synchronous with the peak of the carrier wave sin ωt, and it is possible to detect the sin angle signal (sin θ) corresponding to an envelope curve of the sin signal. In the same manner, it is possible to calculate the cos angle signal (cos θ) from the cos signal. A relation between the sin ωt, the sin θ and the cos θ is illustrated as FIG. 4. Specifically describing the process mentioned above, in order to detect the peak of the carrier wave sin ωt output from the carrier wave oscillation circuit 12, in FIG. 2, a peak value setting circuit 24 indicating a reference amplitude corresponding to the peak value of the sin ωt and the carrier wave signal sin ωt are compared by a comparator circuit 22, and the peak time is detected. It is possible to detect the sin angle signal (sin θ) and the cos angle signal (cos θ) by latching the sin signal and the cos signal in an AD converter 44 and an AD converter 46 serving as a sample hold in synchronous with the peak time.
The sin angle signal (sin θ) and the cos angle signal (cos θ) detected as mentioned above are incorporated into a CPU circuit (an arithmetic processing circuit mainly constituted by a CPU or an MPU), and a process of a flow chart shown in FIG. 3 is executed. In other words, first, the sin angle signal (sin θ) and the cos angle signal (cos θ) are incorporated into the CPU circuit 16 (a step S301), and the CPU circuit 16 executes a calculation of (sin θ)2+(cos θ)2=P (a step S302), and judges normal or abnormal on the basis of whether a value of P exists in a range 0.9<P<1.1 (a step S303).
In Japanese Patent No. 3216491, an abnormality is detected by executing a calculation of (sin θ)2+(cos θ)2=P and judging whether or not P=1 is established (a step S303).
However, whichever method is executed, it is necessary to execute an AD conversion of the sin signal and the cos signal in synchronous with the carrier wave, an interruption by a carrier wave peak timing, a calculation of (sin θ)2+(cos θ)2 and the like, and the calculation requires a lot of processing time. Further, in the case of processing by a software, there is a problem the process is a burden to the CPU, and in the case of processing by a hardware, there is a problem that a lot of hardwares are necessary.
The present invention is made on the basis of the circumstances as mentioned above, an object of the present invention is to provide a control unit for electric power steering apparatus which can detect an abnormality of an angle detecting device of a motor used in the control unit of the electric power steering apparatus, an abnormality of an incomplete disconnection state (a rare short) and the like rapidly, without being a burden to a CPU and without having need of addition of a lot of hardwares.
On the other hand, in the electric power steering apparatus, it is necessary to correctly execute a motor control in such a manner as to output a desired torque in correspondence to a handle operation of a driver. Further, in order to correctly control the motor, it is necessary to detect a state of the electric power steering apparatus by utilizing various sensors. Since a detected signal obtained from the sensor is very important for controlling the electric power steenng apparatus, it is necessary to quickly detect a failure of the sensor so as to execute a control and a protection in correspondence thereto. For example, the control afler detecting the failure of the motor angle detecting device corresponding to one of the sensors is a very important problem for a safety steering of the vehicle, and various control unit have been conventionally considered.
As one example, a description will be given of a control unit in Japanese Patent Application Publication No. 2003-26020 with reference to FIG. 5.
In FIG. 5, a vehicle speed V detected by a vehicle speed sensor 313 and a torque value Tr detected by a torque sensor 107 are input to a torque command value calculating portion 319, and a torque command value Tref is calculated in the torque command value calculating portion 319. Next, the torque command value Tref and a rotation angle θ of a motor 108 output by a resolver digital conversion circuit 311 are input to a current command value calculating means 320, and current command values Idref and Iqref are calculated. On the other hand, if the motor 108 is excited, motor current Ia, Ib and Ic are detected by a current detecting device 312, and the motor currents Ia, Ib and Ic are converted into two-phase currents Id and Iq of d-q axis by a three-phase/two-phase conversion portion 327.
In a subtracting means 321, deviations ΔId and ΔIq between the current command values Idref and Iqref, and the motor currents Id and Iq are calculated. Each of the calculated deviations ΔId and ΔIq is input to a PI control portion 323, voltage command values Vdref and Vqref are calculated in such a manner that the deviation ΔI becomes zero, and the voltage command values Vdref and Vqref are converted into three-phase voltage command values Varef, Vbref and Vcref in a two-phase/three-phase converting means 324. A PWM control portion 325 drives an inverter circuit 326 constituted by FETs on the basis of the three-phase voltage command values Varef, Vbref and Vcref, and supplies the motor current Ia, Ib and Ic to the motor 108. Further, the rotation angle θ of the motor 108 is detected by a resolver 310 corresponding to one example of the angle detecting device and a position detection circuit 311 corresponding to an angle detecting means (the rotation angle θ of the motor 108 is detected mainly by a resolver digital conversion circuit (an RDC circuit), and the rotation angle θ is utilized in the two-phase/three-phase conversion portion 324, the three-phase/two-phase conversion portion 327 and the like.
In the control of the electric power steering apparatus mentioned above, in the case that the detection of the resolver 310 and the resolver digital conversion circuit 311 becomes abnormal, it is impossible to correctly detect the rotation angle θ, and the correct control can not be executed. Accordingly, there is executed the control of stopping the electric power steering apparatus, and changing a handle operation to a manual operated state.
However, if an assist of the handle operation by the electric power steering apparatus is lost, and the operation is changed suddenly to the manual operation, a great uncomfortable feeling is applied to the handle operation. Accordingly, this matter is not preferable. Therefore, in Japanese Patent Application Laid-Open No. 2003-26020, an abnormality judging means 340 is provided. When detecting any abnormality of the resolver 310 or the resolver digital conversion circuit 311, a counter torque against a wish of the handle operation is not generated in some position of the rotation angle θ. Accordingly, there is executed a process of continuing the assist by the electric power steering apparatus for a short time. However, there is a case that it is necessary to immediately stop the electric power steering apparatus in the other position of the rotation angle θ so as to change the handle to the manual operation.
As mentioned above, in the control in the case of detecting the abnormality of the rotation angle θ, there is a problem that the control or the electric power steering apparatus can not be continued in accordance with the position of the rotation angle θ at a time of detecting the abnormality, and the duration is widely limited. Further, the load applied to the driver undesirably becomes greater in the manual operation in correspondence to the increase in size of the vehicle mounting the electric power steering apparatus thereon in recent years.
The present invention is made on the basis of the circumstance as mentioned above, and an object of the present invention is to provide a control unit for electric power steering apparatus which can continue a control of a electric power steering apparatus regardless of a position of a rotation angle θ at a time of failure, even in the case that an angle detecting device for detecting the rotation angle θ of the motor of the electric power steering apparatus is out of order.